1. Field of the Invention
The disclosed technology relates to methods for fabricating thin epitaxial photovoltaic cells.
2. Description of the Related Technology
In order to reduce the costs of crystalline silicon photovoltaic cells, the material consumption of highly pure silicon in a typical photovoltaic cell structure is preferably reduced. Most of the silicon material merely acts as a mechanical carrier for the photovoltaic cell with most of the optical absorption taking place in the upper 30 micrometer.
One approach to reduce the material consumption of highly pure silicon comprises growing a thin crystalline silicon layer of good quality on a low-cost silicon carrier substrate by means of epitaxial growth, wherein the thin epitaxial layer is the active layer of the photovoltaic cell. The carrier substrate can for example be a low-cost substrate which, because of the doping level and/or the impurity level, does not allow the realization of a photovoltaic cell with good conversion efficiency in the substrate. The carrier substrate mainly provides mechanical stability to the photovoltaic cell. However, the supply of such low-cost substrates is today a bottleneck.
In EP0797258, a method is described for fabricating thin silicon photovoltaic cells, wherein the active layer of the photovoltaic cells is epitaxially grown on a reusable, single crystalline silicon substrate. A multi-layer porous structure including two or more porous layers having different porosities is formed in a surface portion of a crystalline silicon substrate, by means of an anodization process. A thin silicon film is epitaxially grown on the porous structure, the silicon film comprising multiple layers forming a solar cell structure (for example a p+ type layer, a p− type base layer and an n+ type emitter layer). After providing for example a surface passivation layer and front side metal contacts, a support substrate is bonded onto the solar cell structure. Next an external force (outward pulling) is applied between the support substrate and the silicon substrate, thereby inducing a tensile stress, such that the solar cell structure (including the support substrate) is separated from the silicon substrate along a line of weakness in the porous structure. After that, a metal film is provided on the back surface of the silicon film as a back surface electrode. The silicon substrate can be reused.